Conventionally, as a high frequency power amplifier that amplifies a modulation signal including envelope fluctuation components, class-A or class-AB linear amplifiers have been used to amplify envelope fluctuation components in linear. Such a linear amplifier excels in linearity, however always consumes power accompanied with direct current bias components, and therefore power efficiency is low as compared with a nonlinear amplifier such as class-C and class-E. Therefore, when such a high frequency power amplifier is applied to a portable radio apparatus in which power is supplied by batteries, since the power consumption of the high frequency power amplifier is large, operating time becomes short. Furthermore, when such a high frequency power amplifier is applied to a base station apparatus of a radio system in which a plurality of large power transmission apparatuses are located, the apparatus becomes large and calorific power increases.
Consequently, as a high efficiency transmission apparatus, a transmission apparatus using a polar modulation scheme is proposed. As shown in FIG. 1, the transmission apparatus using a polar modulation scheme has amplitude phase separation section 10, amplitude modulation signal amplifier 11, frequency synthesizer 12, and high frequency power amplifier 13 which is a nonlinear amplifier.
Amplitude phase separation section 10 receives baseband modulation signal S1 and separates this signal into baseband amplitude modulation signal S2 and baseband phase modulation signal S3. Baseband amplitude modulation signal S2 is supplied to nonlinear high frequency power amplifier 13 via amplitude modulation signal amplifier 11 as a supply voltage of high frequency power amplifier 13. Baseband phase modulation signal S3 is inputted to frequency synthesizer 12. Frequency synthesizer 12 phase modulates a carrier signal with baseband phase modulation signal S3 and thereby obtains high frequency phase modulation signal S4 and transmits this signal to high frequency power amplifier 13. By this means, high frequency power amplifier 13 amplifies high frequency phase modulation signal S4 at the supply voltage according to baseband amplitude modulation signal S2 and outputs the result as transmission output signal S5.
Next, the operation of the transmission apparatus using a polar modulation scheme will be described. First, if baseband modulation signal S1 is Si(t), Si(t) can be expressed in the next equation.
[Equation 1]Si(t)=a(t)exp[jφ(t)]  (1)
Here, a(t) is amplitude data, and exp[jφ(t)] is phase data.
Amplitude phase separation section 10 extracts amplitude data a(t) and phase data exp[jφ(t)] from Si(t). Here, amplitude data a(t) corresponds to baseband amplitude modulation signal S2, and phase data exp[jφ(t)] corresponds to baseband phase modulation signal S3. Amplitude data a(t) is amplified at amplitude modulation signal amplifier 11 and provided to high frequency power amplifier 13. By this means, the supply voltage value of high frequency power amplifier 13 is set based on amplitude data a(t).
Frequency synthesizer 12 generates high frequency phase modulation signal S4 in which carrier angular frequency ωc is modulated at phase data exp[jφ(t)], and the result is inputted to high frequency power amplifier 13. Here, if high frequency phase modulation signal S4 is Sc, Sc can be expressed in the next equation.
[Equation 2]Sc=expj[ωc×t+φ(t)]  (2)
Then, by using a nonlinear amplifier as high frequency power amplifier 13, transmission output signal S5, in which a signal that multiplied supply voltage value a(t) of high frequency power amplifier 13 and an output signal of frequency synthesizer 12 together is amplified by gain G of high frequency power amplifier 13, can be obtained. Here, suppose transmission output signal S5 is RF signal Srf, RF signal Srf can be expressed in the next equation.
[Equation 3]Srf=Ga(t)Sc=Ga(t)expj[ωc×t+φ(t)]  (3)
A signal inputted to high frequency power amplifier 13 is a phase modulation signal which does not have fluctuation components directed to amplitude and therefore is a constant envelope signal. Consequently, it is possible to use a nonlinear amplifier which is efficient as high frequency power amplifier 13, so that a transmission apparatus with high efficiency can be provided. This kind of technology using polar modulation is disclosed in, for example, Patent Document 1 and Patent Document 2.
Patent Document 1: Japanese Patent Publication Laid-Open No. 3207153
Patent Document 2: Japanese Patent Application Laid-Open No. 2001-156554